The present invention is related to thermal printing systems and, more particularly, to a method of reducing printhead switching to increase printing speed and extend printhead life.
Thermal printing systems are used to print images on substrates using a thermal printhead and a thermal print ribbon that is positioned between the printhead and the substrate. The printhead is used to heat the thermal print ribbon and cause print material (black or colored) to transfer to the substrate and form the desired image.
The thermal printhead generally includes resistive heating elements, which are uniformly deposited in a single line and are positioned closely together, typically with a resolution of 200 or 300 resistive elements per inch. Each of the resistive elements corresponds to individual pixels of an image line, several of which are printed to form the image. A strobe signal, generated by a controller, switches a power supply that applies a current to the resistive elements, which are enabled in accordance with data that is latched into a burn register of the printhead. The current energizes the enabled resistive elements causing them to heat the thermal print ribbon. This process of energizing the resistive elements is generally part of a burn cycle, at least two types of which are used to print an image line. These include a pre-burn cycle and a print material transfer burn cycle.
The pre-burn cycle is first performed to preheat the resistive elements to a threshold level, above which print material from the thermal print ribbon begins to transfer to the substrate. The print material transfer burn cycle is performed to heat enabled resistive elements beyond the threshold level to thereby cause print material to transfer from the thermal print ribbon to the substrate. These burn cycles involve first loading (clocking) data into a shift register of the printhead, latching the data into the burn register to enable or disable individual resistive elements, and activating the power supply of the printhead to apply current to the enabled resistive elements for a pre-determined period of time. Once the pre-determined period of time has expired, the strobe deactivates the power, new data is then loaded into the shift register and latched into the burn register, and the strobe signal reactivates the power to the enabled resistive elements again for another pre-determined period of time. This step is repeated numerous times in accordance with the particular type of burn cycle. As a result, the power supply of the printhead is switched several times along with the enabled resistive elements.
This frequent switching of the resistive elements and the power supply is undesirable. Each voltage pulse produced by the power supply causes stress on the resistive elements and the electronics of the printhead, which can cause them to degrade and reduce the operable life span of the thermal printhead. Further, the non-continuous heating of the resistive elements results in a slow printing process. Further still, the amplitude of the voltage and current that is applied to the resistive elements is typically high in order to compensate for heat losses caused by the frequent switching and to increase printing speed. Consequently, these methods of performing a burn cycle in a thermal printer cause significant wear to the thermal printhead.
There exists a need for an improved method of performing a burn cycle that reduces printhead switching while increasing printing speed and extends printhead life.
The present invention is directed toward a method of performing a burn cycle in a thermal printer that reduces printhead switching, increases printing speed, and extends printhead life. In the method, data is loaded into a shift register of a thermal printhead to designate resistive elements that are to be enabled and disabled during the burn cycle. Next, the data is latched into a burn register of the printhead and a power supply of the printhead is activated thereby energizing the enabled resistive elements. New data is then loaded into the shift register. After a short burn period has expired, the new data is latched into the burn register. The steps of loading and latching new data are repeated a predetermined number of times, after which the power supply is deactivated to complete the burn cycle. The present invention is further directed toward a thermal printer that is adapted to implement the above-describe method.